It is well known and frequently practiced in the art to stretch a polymeric material of continuous length such as a synthetic fiber to effect molecular orientation therein to improve physical properties such as mechanical strength, etc. Generally speaking, however, in many polymeric materials, improvement of physical properties such as tensile strength, tensile modulus, flexural strength, buckling strength, etc. cannot sufficiently be effected even if they may be subjected to stretching. For example, there has been proposed, for making a higher modulus of polyacetal, a two-step stretching method, in which polyacetal is stretched twice while being heated under normal pressure (Poly. Eng. & Sci., Vol. 14, No. 10, p. 682). This method, however, is not practically applied because of the low maximum value of the modulus reached, generation of voids and fibrils and the drawback of its difficult applicability to rods or tubes with larger diameter. The microwave heating stretching method has also recently been proposed (Japanese Laid-open Patent Publication No. 208216/1982), but generation of voids and fibrils could not be prevented and the modulus attained was not satisfactory. As one method for inhibiting voids accompanied with such stretching, it may be considered to carry out working under a high hydrostatic pressure. For example, it was attempted to place a test piece of polyacetal in a sealed vessel and stretch at room temperature under a high hydrostatic pressure, but fracture occurred before reaching the yield point, or the product obtained by discontinuing stretching before fracture was found to have no improved physical properties (Poly. Eng. & Sci., Vol. 8, No. 4, p. 290). It has also been known to extrude polyacetal under hydrostatic pressure and perform stretching through a dice (J. Appl. Polym. Sci. Vol. 26, p. 2879), but formation of voids and fibrilation occurs during stretching, so that no high strength and high modulus can be attained
As another proposal, a test piece of polyacetal or other plastics previously stretched to such a low extent as to effect molecular orientation was placed in a sealed vessel, heated to a temperature higher than the softening point thereof and pressurized under hydrostatic pressure, thereby improving thermal stability and transparency (Japanese Laid-open Patent Publication No. 141371/1978). According to this method, no high stretching is possible to thus make it impossible to attain a high modulus. Moreover, limitations with respect to the shape of the applicable molded article cannot be avoided.
Attempts have also been made to improve the physical properties of polyacetal or polyethylene, by placing such a polymer in a sealed vessel and carrying out dice extrusion by application of high hydrostatic pressure from one end (J. Appl. Polym. Sci., Vol 26, 2877-2896; J. Polym. Sci., Vol. 16, 2031-2047; J. Mater. Sci., Vol. 10, 1105). However, the product obtained is low in modulus attained even by high stretching, and a high pressure of several tons/cm.sup.2 or higher is required. Besides, the process has a low extrusion speed and is batch, and thus it can hardly be deemed practical.
In addition, although the working method under hydrostatic pressure of the prior art as described above, which is the treatment in a laboratory to apply a pressure treatment batchwise on a test material in a sealed system, can be easily performed with relative ease without taking out the test material, it has not yet been realized in an operation of a large scale production where a material of continuous length must be treated continuously, because of the difficulty in a device to provide openings for the introducing and withdrawing of the material in continuous length and to maintain the pressure internally applied.
The present inventors have made various studies to determine the causes for insufficient improvement of tensile strength and tensile modulus in the methods of the prior art. Consequently, it has now been found that, in most polymeric materials, high stretching to the extent so as to give high modulus cannot be done, and yet simultaneously with the orientation of the polymers at stretching generation of voids and fibrilation actually occurs, whereby the polymers tend to be readily bursted in the stretched direction to be lowered not only in mechanical strength in the stretched direction, but also in flexural strength and buckling strength in the transversal direction. Accordingly, it has been found that marked improvement can be made only if stretching can be performed at a high stretching ratio while preventing generation of voids and fibrilation. On the basis of this knowledge, further studies were continued for development of a method for stretching a polymeric material to a high ratio so as to achieve a high modulus without fibrilation, to find that the purpose can be achieved by stretching while applying direct pressure with a fluid and heating and to invent a device for carrying out the process continuously, thus obtaining the present invention.